Siderostat, any of a class of astronomical instruments consisting of a flat mirror that is turned slowly by a motor to reflect a given region of the sky continuously into a fixed telescope. In the traditional siderostat, the mirror is rotated by a lever arm connected to a motor that turns at a rate of one revolution every 24 hours. This so-called Foucault siderostat provides a fixed but rotating image. In recent years the Foucault siderostat has been largely supplanted by the heliostat, which is a polar siderostat, and by the coelostat.

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instrument used in solar telescopes to orient and focus sunlight along a fixed direction. A typical heliostat consists of a flat plane mirror and a curved parabolic mirror. The plane mirror is mounted along an axis parallel (i.e., equatorial) to Earth and rotated slowly by a motor to reflect light...

device consisting of a flat mirror that is turned slowly by a motor to reflect a portion of the sky continuously into a fixed telescope. The mirror is mounted to rotate about an axis through its front surface that points to a celestial pole and is driven at the rate of one revolution in 48 hours....